One of the starting moves for Druids in Dungeon World 1 is:

Shapeshifter

When you call upon the spirits to change your shape, roll+Wis.

✴ On a 10+ hold 3.

✴ On a 7–9 hold 2.

✴ On a miss hold 1 in addition to whatever the GM says.

You may take on the physical form of any species whose essence you have studied or who lives in your land: you and your possessions meld into a perfect copy of the species’ form. You have any innate abilities and weaknesses of the form: claws, wings, gills, breathing water instead of air. You still use your normal stats but some moves may be harder to trigger—a housecat will find it hard to do battle with an ogre. The GM will also tell you one or more moves associated with your new form. Spend 1 hold to make that move. Once you’re out of hold, you return to your natural form. At any time, you may spend all your hold and revert to your natural form.

At one point I considered this the most broken move in Dungeon World, granting so much potential power to players, but I grew to love it. 

Managing the hold can be troublesome. (Note that Jeremy Strandberg removed hold altogether from Homebrew World’s take on this move: Shapechange.) After the player spends the final hold, as a complication on misses and mixed successes, I sometimes had them half-transform back to human form.

In one of my campaigns, the Fighter took Multiclass Dabbler and then Shapeshifter as well as the ability to Cast a Spell. He said he was learning from his two companions (Druid and Wizard). 

His first Shapeshifter roll was 6-, and he did not roll great on it other times. He just was pretty bad at shapeshifting, as luck would have it. As a result of these mishaps, he had huge—now-flightless—wings instead of arms. He was a human with the head of the magpie (so he couldn’t talk). A human but with magpie legs. I played it that his maltransformation affects his next move in the fiction, then fixes itself.

As a result of all this, he grew to believe he’s a magpie with the ability to turn into a human! (His idea.) And he had nightmares that one day he would not be able to leave human form (because when he tries to “return” to magpie form he’s not always able to stay in that form)!

At the end of the campaign, when he retired, he stayed in magpie form and went to live in an aviary!

Photo by Daniel Bertrams on Unsplash.